Conventionally, machinery parts such as a gear or the like are worked by utilizing metal cutting machines such as gear cutting machine or the like because it is required for the parts to have a high accuracy. However, according to this method, much time is required to work the parts, so that productivity is disadvantageously lowered.
To improve such disadvantages, there has been proposed and actually used a method of manufacturing a small-sized gear by utilizing a forging method. However, in a case where the gear is manufactured by the forging method, many processes such as extruding, upsetting of raw material, ironing of a tooth surface of the gear, or the like are required, so that a surface pressure to be applied to the tooth surface is increased thereby to cause burning or the like. As a result, there is inevitably posed the disadvantage that molding with high accuracy cannot be performed.
In order to improve such disadvantages, for example, an official gazette of Japanese Patent Laid-open Publication No. HEI 5-154598 has proposed a forging method in which a raw material is forged to form a spur gear without increasing the surface pressure of the tooth portion of the gear.
The method of forging the spur gear disclosed in the official gazette is characterized by comprising: a first working process in which a raw material is upset-molded into a primary worked gear having a gear-shape whose tooth profile is set to be smaller than that of a spur gear to be obtained; a second working process in which the material is freely flowed at portions other than tooth portion of the primary worked gear while the gear is compressively molded into a secondary worked gear; and a third working process in which the secondary worked gear is ironically molded into a final product, and wherein the respective processes are performed in accordance with a cold forging working method.
According to the above method, an ironing allowance for the ironical-molding operation is stably formed, so that there can be provided an effect of obtaining a product having a high accuracy by a low molding load.
Further, as a conventional extrusion working method, there is a method in which the material is extrusion-molded using a punch having a hole portion thereby to form a convex portion from the hole portion of the punch, a free end portion of the convex portion is extrusion-worked under a state of being pressed with a predetermined pressure, and the pressure is reduced or removed before the extrusion-work is completed. This method provides an effect of greatly reducing a maximum load to be applied to the molding die in the vicinity of a bottom dead point of the molding die.
However, in the forging method described in the above mentioned official gazette, since the raw material is molded in advance so as to have a gear-shape whose tooth profile is set to be smaller than that of a spur gear to be obtained, one or two processes are required for molding the raw material to form a gear-shape. In addition, three processes are required for forging the raw material, and the number of processes is increased, so that the forging operation requires much time, providing a disadvantage of lowering the productivity.
In addition, changing the punch for every process, the forging operation is performed, the changing work of the punch becomes to be complicated, and a plurality of punches and mandrels are required to be prepared in advance, so that there is posed a disadvantage of increasing the cost of the molding dies.
On the other hand, in the conventional extrusion working method, the material is subjected to a back-extrusion while a flow of the material is partially suppressed by a pressing member provided to an upper punch, and the pressing force of the pressing member is reduced or removed before the extrusion-work is completed, so that a high surface pressure is applied to the molding die from a time when the molding operation is started. As a result, there may be caused various disadvantages such that the molding die causes burning and a life of the molding die decreases in a short period of time.
In addition, in the conventional extrusion working method described above, since the device for effecting the method has a structure in which the pressing member is provided only to the upper punch side, the pressing forces of the upper and lower punches are not uniform with each other and the material flows in only one direction, so that there may also be posed disadvantages such that a strength of the product is lowered, and defects such as cracks and shrinkage cavity are liable to be caused in the product.
Therefore, the present invention has been achieved for improving such disadvantages, and an object of the present invention is to provide a forging die device and upset-forging method capable of forging parts such as a gear having a high accuracy without requiring the pre-working of the raw material and the change of dies during the forging, thereby to improve the productivity and to decrease the cost of the molding dies.